A maximum contaminant level (MCL) is the highest concentration of a contaminant allowed in public drinking water under the Safe Drinking Water Act. MCLs are legally enforceable standards set by the EPA.
Maximum contaminant levels (MCLs) are enforceable standards established by the U.S. Environmental Protection Agency under the Safe Drinking Water Act that define the maximum permissible concentration of a contaminant in water delivered to any user of a public water system. MCLs are set as close to the maximum contaminant level goal (MCLG) as technologically and economically feasible, where MCLGs represent the level at which no known or anticipated health effects occur with an adequate margin of safety. The EPA currently regulates over 90 contaminants under the National Primary Drinking Water Regulations, including microorganisms, disinfectants, disinfection byproducts, inorganic chemicals, organic chemicals, and radionuclides. Public water systems must regularly test for regulated contaminants and report results to their state primacy agency and to consumers through annual Consumer Confidence Reports. Violations of MCLs can result in enforcement actions, mandatory public notification, and required corrective measures. MCLs are periodically reviewed and updated as new scientific evidence emerges about health effects and as treatment technologies advance. States may adopt MCLs that are more stringent than federal standards.